A calmodulin-dependent cyclic nucleotide phosphodiesterase from bovine brain supernatant has been purified by means of a new procedure, termed "sequential adsorption-electrophoresis." This method provides rapid and extensive purification (2000-3000 x) in high yield (30-40%) and may be applicable to other calmodulin-binding proteins. The basal enzyme activity is inhibited noncompetitively by physiologic concentrations of polyamines, while Ca ions-dependent activity is unaffected. Spermine stabilizes activity of purified enzyme and appears to antagonize the stabilization provided by MgCl2. The polyamines may compete for a Mg-binding site related to both stability and basal" activity.